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Last updated October 2026

Sonoff WiFi Smart Plug S40

A 15A Wi-Fi plug that tracks ghost power and EV charging usage well and works smoothly with Home Assistant, though a share of owners report units failing after a few months.

4.2
77 ratings
$10.90
As an Amazon Associate I earn from qualifying purchases
Launch Year: 2021

Model: S40TPB

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Our review

Sonoff WiFi Smart Plug S40 Review: Solid Power Monitoring With Some Longevity Concerns

The verdict

The Sonoff S40 gives buyers real-time and historical energy monitoring in an affordable Wi-Fi plug, and owners use it for everything from tracking standby power draw on electronics to monitoring home EV charging sessions. It integrates well with Home Assistant and voice assistants, and its firmware is upgradeable with configurable startup behavior. The main risk buyers should weigh is a recurring pattern of units losing Wi-Fi connectivity or failing outright after a couple of months, alongside some reports of setup friction and a power app whose usage graphs are not very granular. For anyone wanting basic energy insight at a fair price and willing to accept some unit-to-unit reliability variance, it is a reasonable pick.

Best for
  • Home Assistant users
  • Ghost/standby power tracking
  • EV charging monitoring
  • Budget buyers

Pros

  • Real-time and historical energy monitoring
  • Handles up to 1800W/15A loads with overload protection
  • Works well with Home Assistant, Alexa, Google Home and SmartThings
  • Upgradeable firmware with configurable power-on behavior
  • Useful for tracking ghost power and EV charging

Cons

  • Some units reported losing Wi-Fi connectivity or failing after a few months
  • Energy readings can be inaccurate at very low power draws
  • Usage history graphs are only monthly, with no min/max/average view
  • No Zigbee; Apple Home and Matter only through a SONOFF hub
  • No USB charging port or weather resistance

Who is the Sonoff WiFi Smart Plug S40 for?

Owners use this plug to measure how much power electronics like TVs draw even when turned off, to monitor home EV charging sessions, and to run higher-wattage appliances such as space heaters on a schedule. It is also popular with Home Assistant users, who report getting it working reasonably quickly by following the manufacturer's setup instructions.

What buyers love

The most consistent praise is for the plug's energy monitoring, which owners use to see real-time power draw plus daily, weekly and monthly history through the app, matching its supported energy-monitoring feature. Reviewers also value its ability to handle bigger loads reliably, its upgradeable firmware, and configurable power-on behavior after an outage. Home Assistant users specifically call out good support and a working eWeLink app experience.

What to know before you buy

A notable share of reviewers reported units that stopped connecting to Wi-Fi or failed outright after two to three months of use, so build quality can vary between units. A few buyers also found initial pairing difficult. One detailed review noted the app's energy readings can be inaccurate at very low power draws (sometimes showing 0W) and that the historical usage graphs are only available at a monthly resolution with no min/max/average breakdown. It also lacks Zigbee support, reaches Apple Home and Matter only through a SONOFF hub that bridges it, and has no USB charging port or weather resistance.

Is the Sonoff WiFi Smart Plug S40 worth it?

With a 1800W/15A rating, overheat and overload protection, real energy monitoring, and support for Google Home, Alexa, SmartThings, IFTTT and Home Assistant, this plug offers a strong feature set for its price. Sentiment is generally positive on functionality and monitoring detail, tempered by a real minority of buyers who experienced early failures, so it suits buyers comfortable with typical budget-electronics reliability trade-offs rather than those who need guaranteed long-term uptime.

Frequently asked questions

Does the Sonoff S40 monitor energy usage?

Yes, it provides real-time power monitoring plus daily, weekly and monthly usage history in the app.

Does it work with Home Assistant?

Yes, reviewers report it integrates well with Home Assistant, alongside support for Google Home, Alexa, SmartThings and IFTTT.

Is the Sonoff S40 reliable long-term?

Most reviews are positive, but a meaningful number of buyers reported units losing connectivity or failing after a couple of months of use, so reliability can vary by unit.

What is the maximum load the Sonoff S40 can handle?

It is rated for up to 1800W and 15A, with overheat and overload protection built in.

Does the Sonoff S40 support Apple Home or Zigbee?

Not directly: it has no Zigbee, and Apple Home and Matter are only reachable through a SONOFF hub that bridges it. It works with Google Home, Alexa, SmartThings, IFTTT and Home Assistant.

Company
Sonoff logo

Sonoff

4.3/ 5 avg ratingChina flagHeadquartered in China

Sonoff, a brand of ITEAD, makes affordable smart home devices including Wi-Fi and Zigbee switches, smart plugs, relays, sensors and the ZBBridge and Zigbee dongles. Its products work through the eWeLink app and integrate with Alexa and Google Home, and are popular with DIY users and Home Assistant fans for low-cost automation. Sonoff is well known for budget-friendly switches and Zigbee hardware.

Full specifications

Every spec, organized

Browse the full breakdown by category. Tap the on any row for what it means, or the for sources and documentation.

Weather Resistance

NO

Dimensions

3 × 1.6 × 1.3 in 76 × 40 × 32 mm

Weight

2.6 oz 74 g

Available in colors

NO

Plug Format

Type B

Maximum Load Power

1800W

Maximum Load Current

15A

Load Voltage

120V

Overheat/Overload Protection

YES

Energy Monitoring

YES

USB Charging

NO

2.4 GHz WiFi

YES

5 GHz WiFi

NO

Bluetooth

For Pairing

Z-Wave

NO

Zigbee

NO

Thread Protocol

NO

Google Home

YES

Alexa

YES

Apple HomeKit

SONOFF Hub required

SONOFF lists this model among the eWeLink Wi-Fi devices its Matter bridges (Zigbee Bridge Ultra, iHost, NSPanel Pro) publish to Matter, which is how it reaches the Apple Home app.

Samsung SmartThings

YES

Home Assistant

YES

Matter

SONOFF Hub required

The device is not Matter itself: SONOFF lists it among the eWeLink Wi-Fi models its Matter bridges (Zigbee Bridge Ultra, iHost, NSPanel Pro) republish into Matter.

On/Off Button

YES

Zigbee Router

NO

Automations

YES

Local Control

YES

eWeLink lists the S40TPB among the Wi-Fi devices with LAN control that its local eWeLink CUBE hub integrates.

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